Stretch/cling films have found utility in a wide variety of fields including the bundling and packaging of food and other goods. One application of particular, but not limiting, interest to the present invention is in the bundling of goods for shipping and storage such as, for example, the bundling of large rolls of carpet, fabric or the like for shipping from the manufacturer to a retail outlet. An important subset of these bundling applications is in the containment and unitizing of pallet loads.
The load of a pallet may be unitized or "bundled" by stretch-wrapping a film several times around the articles to be palletized. There exist a variety of stretch-wrapping techniques, two of which are commonly employed. In one technique, the loaded pallet is placed on a rotating turntable and the end of a continuous roll of film attached to the load. As the turntable rotates, the film is continuously wrapped around the pallet and load. Tension is applied to the film roll to cause the film to stretch as it is applied.
Because the film is in a stretched condition, it is placed under considerable tension and will have a tendency to return to its original, unstretched state. This tension can cause the film to unravel from the wrapped pallet, thereby jeopardizing the integrity of the unitized load. It is desirable, therefore, that the film have cling properties to prevent unraveling of the film from the pallet. The tension in the stretched film may also cause the film to be more susceptible to punctures and tears. It is, therefore, also desirable for the film, as a whole, to have good stretch, tensile, puncture resistance and tear resistance properties. Thermal stability of the various film components is important for the recycling of edge trim and film scrap generated in the various film production processes.
To impart cling properties to, or improve the cling properties of, a particular film, a number of well-known tackifying additives have been utilized. Common tackifying additives include polybutenes, terpene resins, alkali metal stearates and hydrogenated rosins and rosin esters. The use of tackifying additives, however, may not be desirable. These additives have a tendency to accumulate on the stretch wrapping apparatus often requiring additional periodic cleaning and maintenance. They also can migrate into the bundled or unitized articles resulting in damage to such articles, as well as migrating throughout a film, even a multilayer film, causing tackiness on both sides. In palletizing operations, this may cause the film on adjacent pallets to cling together resulting in tear, puncture or other damage to the wrap and jeopardizing the integrity of the unitized load.
For this reason, it is desirable for the film to have slip or "anti-cling" properties on its "outer" side to prevent this interpallet cling. Slip is defined in terms of coefficient of friction. In other words, it is desirable that the "outer" side of the film have a low coefficient of friction in contact with another object, particularly another like film. As with cling, slip can be imparted to the film or improved through the use of various well-known slip, anti-cling and/or antiblock additives including silicas, silicates, diatomaceous earth and various lubricants.
It is also a requirement that the film can be unwound from rolls in which it is often shipped and stored until ready for use, for example, in palletizing operations. Over a period of time, particularly at elevated ambient storage temperatures, the film can block or fuse together in adjacent layers so as to inhibit unwinding by the adjacent layers sticking together, and in severe cases of destructive block, actual tearing or delamination of the film can occur. The conventional anti-cling and slip additives also generally serve to inhibit blocking.
In addition to conventional anti-cling additives, film cling properties can be altered by exposure to a corona discharge. The reactivity of discharge particles can alter the surface chemistry of the film and consequently its physical characteristics. The effect of such exposure can be observed by changes in film surface energy.
A wide variety of thermoplastic polymers such as, for example, polyvinyl chloride, polyethylene, polypropylene and various polymers of ethylene and other comonomers, most notably vinyl acetate, have been used as stretch/cling films. These materials standing alone, however, suffer from a number of shortcomings. Most do not possess desirable cling properties without the use of tackifying additives. Further, most cannot be stretched to a great extent without adversely affecting their cling, slip, tensile, tear resistance and puncture resistance properties. For the particular case of ethylene-vinyl acetate polymers, thermal stability becomes a problem on the reprocessing of trim and scrap.
More recently, the use of multilayer films has gained popularity. With a multilayer film, one can obtain a stretch/cling wrap having cling properties on one side and slip properties on the other side. For example, U.S. Pat. No. 4,518,654 discloses a multilayer film having an A/B construction wherein the A side has cling characteristics and the B side has slip characteristics. In the aforementioned patent, the A side is said to comprise a polyethylene or an ethylenemonoolefin polymer, preferably linear low density polyethylene (LLDPE). To provide the LLDPE with the desired cling properties, a tackifying agent is added to the polymer. The B side is said to comprise a low density polyethylene (LDPE) with an anticling additive added to impart the desired slip properties to the LDPE. This patent is hereby incorporated by reference herein for all purposes as if fully set forth.
Other multilayer films comprising layers of the various aforementioned stretch/cling materials are disclosed in U.S. Pat. Nos. 3,508,944, 3,817,821, 4,022,646, 4,082,877, 4,147,827, 4,189,420, 4,194,039, 4,303,710, 4,399,180, 4,364,981, 4,418,114, 4,425,268, 4,436,788, 4,504,434, 4,588,650 and 4,671,987; U.K. Patent Application 2,123,747; French Patent 2,031,801; and European Patent Application No. 0,198,091, all of which are also incorporated by reference herein for all purposes. These multilayer films are generally produced by one of a number of well-known coextrusion processes also disclosed in the aforementioned incorporated references.
Many of the multilayer films, however, still suffer from shortcomings possessed by their individual layers. For instance, most still require the use of tackifying additives to impart cling properties to the film. As previously mentioned, these tackifying additives can have a tendency to accumulate on dispensing apparatus and may migrate through the films to the slip side. Also, films containing a tackifying additive may be prepared and used in such a manner that the tackifying additive is "picked off" and onto the slip side of the film because the slip and cling layers of the film are in intimate contact on the film roll. Others do not possess desired slip properties, particularly when in a highly stretched state. Still others do not possess a desirable combination of stretch, tensile, tear resistance, puncture resistance and thermal stability properties.
It is known to treat various polymer surfaces by fluorination. Materials like polytetrafluorethylene (PTFE) have been used extensively and have very low polar surface energies. As an alternative to fluorinated polymers like PTFE which are relatively expensive, localized fluorination at the polymer surface has been used to reduce cost.
U.S. Pat. No. 3,740,325 to Maulon discloses a material and process for making a surface fluorinated material. The treated material is said to have greater water resistance, corrosion and soil resistance and be more like PTFE.
U.S. Pat. No. 3,647,613 to Scotland discloses a surface fluorinated high density polyethylene used for making containers suitable for storing hydrocarbons. The HDPE surface containing a concentration of 0.01 to 30 micrograms of fluorine per square centimeter was said to improve permeability characteristics to gasoline.
U.S. Pat. No. 4,880,675 discloses plastic containers comprising a polyethylene inner layer coextruded with a polypropylene outer layer wherein the inner layer is treated with a reactive fluorine source. Such treatment is said prevent the absorption of flavor or aroma compounds by the container lining.
U.S. Pat. No. 4,743,419 to Bierschenk discloses an on-line method for continuous fluorination of a polymer film while the film is being extruded.
U.S. Pat. Nos. 4,264,750 and 4,404,256 to Anand et al. disclose a process for making fluorinated low energy polymer surfaces by exposure to ions or radicals of fluorinated species in a cold plasma and polymers having a substantially completely fluorinated surface of low surface energy which is substantially oxygen free.
Surface fluorination of high density and low density polyethylene films between 40.degree. C. and 100.degree. C. is described by Volkmann et al. in Makromolecular Chemie, Macromolecular Symposium, Vol. 25, pp. 243-248 (1989). The reaction is said to be diffusion controlled with the degree of fluorination increased by greater F.sub.2 concentration in the fluorinating gas mixture.
Processes for surface fluorinating polymer films have been observed to introduce oxygen into the surface as a reaction byproduct when reaction times are smaller than those required to achieve complete surface fluorination. Anand et al. attributes this fact to a post-reaction with atmospheric oxygen or oxygen impurities possibly due to radicals trapped in the subsurface or some unreacted bonds on the surface. At any rate, this phenomenon is not generally observed following complete surface fluorination.
The presence of oxygen following a partial fluorination of a polymer surface increases surface energy, particularly the polar component, making the surface more adherent. Milker and Koch, "Surface Pretreatment of Polymer webs by Means of Fluorine; and Milker and Koch, "Fluorine Makes Plastics Flexible"; describe a fluorination process for producing polymer films said to have good adhesion properties. The process is said to alter the surface on a more or less permanent basis so that the polar character of the material is increased.